The Viperidae (vipers) are a family of venomous snakes found all over the world, except in Oceana, Antarctica, Ireland, Madagascar and north of the Arctic Circle. They have relatively long, hinged fangs that permit deep penetration and injection of venom. The fangs are at the front of the mouth, attached to a mobile maxilla, enabling the fang to fold away against the roof of the mouth, thus permitting longer fangs compared to head size. A detailed description of this family can be found here.